improve filter form

- hide form in some places ;
- i18n
- hide status filtering with a parameter (redundant)

Form/SingleTaskType.php

@@ -49,7 +49,8 @@ class SingleTaskType extends AbstractType
             ->add('assignee', UserPickerType::class, [
                 'required' => false,
                 'center' => $options['center'],
-                'role'   => $options['role']
+                'role'   => $options['role'],
+                'placeholder' => 'Not assigned'
             ])
             ->add('circle', ScopePickerType::class, [
                 'center' => $options['center'],